Heres a good article talking about how accurate various games are with the real thing.
http://www.seight.com/ring-sims.html
Very nice article
By driving a car round at a constant speed (manual gears, 2nd gear on the limiter) you soon find out that it is over 17 miles long instead of the 13.04 that the game claims! Repeat the same test in GT4 and the circuit length is spot on.
Seems that FM3 might have same track models with bit more detail... So a 17 mile NUR is a NO NO. And GT4 is SPOT on.. amazing.. if only in GT4 you can go bit slower..